There are several applications where the exclusive excitation of the fundamental mode of a multimode waveguide structure is advantageous. For some fiber Bragg grating (FBG) sensor applications, refractive index modulated distributed Bragg gratings are fabricated in multimode waveguides, for example in sapphire fiber rods as disclosed by Grobnic et al in IEEE Photon. Technol. Lett. 16 (11), p. 2505-2507 (2004) incorporated herein by reference. It is desirable to obtain a single mode response in the reflection spectrum rather than a multimode response, as the spectral bandwidth of the single mode response is narrower. An improved signal to noise ratio results when only one mode is being inspected rather than the superposition of hundreds if not thousands of modes that can be supported by the multimode fiber.
As well, expanding the mode field diameter (MFD) of a single-mode fiber (SMF) has many advantages in terms of reducing the local power density while increasing the effective free-space collimation distance (or ‘Rayleigh range’). Reducing the local power density allows for the transmission of higher power densities between optical components while reducing the possibility of fiber damage at coupling points. The capability of reducing local power densities at coupling points is important for multi-component fiber optic systems.
For high-powered fiber lasers that produce in excess of 1 kW average power, large mode area (LMA) fibers are required so that the optical intensities achieved are below the damage threshold of the material. As LMA fibers are multimode waveguide structures, it is desirable to have light propagate in the fundamental mode of the waveguide in order to produce high quality diffraction limited beam output. This is typically achieved by using LMA fibers that support the propagation of only a few modes wherein excitation of higher order modes is suppressed.
Several technologies for expanding the fundamental mode field of a single-mode optical fiber have been developed: for example the coupling of microlenses such as graded-index fiber lenses (GRIN) as disclosed by Emkey et al. in J. Lightwave Technol. 5, pp. 1156-1164 (1987) incorporated herein by reference; diffusion of core dopants, also known as Thermally Expanded Core (TEC) techniques as disclosed by Hanafusa et al. in Electron. Lett. 27 (21), p. 1968-1969 (1991) incorporated herein by reference; residual elastic stress based approaches as taught by DiGiovanni et al in U.S. Pat. No. 6,768,849; ‘free-space’ beam expansion via SMF coupling to cladding matched materials as taught by Duck et al in US Patent Application 200310103724 A1; and by physical tapering of SMFs as disclosed by Jedrzejewski et al. in Electron. Lett. 22 (2), pp. 105-106 (1986) and Amitay et al in U.S. Pat. No. 4,737,004.
These prior art single mode expansion techniques provide a useful function, however they are known to suffer from some limitations depending upon their application. Mode expansion techniques that require the inclusion of a microlens to collimate the expanded beam, as described by Emkey or Duck, need to have stable and critical alignment in order to reduce coupling losses of the expander. In the case of GRIN lenses, precise lens lengths are also required to obtain the correct focal length that expands the fundamental mode field of a SMF. The optical characteristics of GRIN lenses are extremely sensitive to their length so fabricating connectors from GRIN lens beam expanders requires careful control of the fiber position in the ferrule. Diffusion of the core dopants has been shown to expand the size of the fundamental mode of SMF but requires extremely high temperatures and very long processing times. Such high temperature processing can weaken the fiber or distort its shape.
Mode field expansion through tapering of SMF, as disclosed by Jedrzejewski and Amitay, eliminates the need for lenses. For step index fibers the normalized frequency or V number is given by:
                    V        =                                            2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              r                        λ                    ⁢                                    (                                                n                  co                  2                                -                                  n                  ci                  2                                            )                                                          (        1        )            
where r is the core radius, λ is the wavelength and nco and ncl are the refractive indices of the core and cladding respectively. For single mode operation, V≦2.405. When a fiber is tapered, by using the hydrogen flame brushing technique for example (see Bilodeau et al U.S. Pat. No. 4,895,423 incorporated herein by reference) the ratio of cladding/core radii remains constant however V decreases. As disclosed in Love et al IEE Proceedings Journal vol. 138, no. 5, p. 343-354 (1991), incorporated herein by reference, when single mode optical fiber is tapered down such that the normalized frequency or V number of the taper is V<0.84, the fundamental LP01 mode is no longer confined to the core but instead is guided by the cladding-air interface resulting in a mode field with the same diameter as the tapered fiber.
In order to minimize coupling of the fundamental mode into higher modes within the taper, the tapering rate along the length of the fiber must be adiabatic. If z is the distance along the taper and ρ is the local taper radius then the adiabatic condition can be written as:
                                                                                    ⅆ                ρ                                            ⅆ                z                                                          ≤                      ρ                          z              b                                      ⁢                                  ⁢                              z            b                    =                                    2              ⁢                                                          ⁢              π                                      (                                                β                  1                                -                                  β                  2                                            )                                                          (        2        )            
where zb is a ‘beat length’, β1 is the propagation constant of the fundamental LP01 mode and β2 is the propagation constant for the LP02, which is the closest and most likely mode to which coupling will occur. There are limitations to the amount of mode field expansion that is possible by ‘down’ tapering existing commercially available SMF, typically a factor of 5 to 10.
Considering sensors in fiber rod waveguides such as sapphire fiber, (for example fiber Bragg gratings, Fabry-Perot based sensors), in order to maximize the coupling of the fundamental mode (LP01) from the beam expander to the sapphire rod, it is necessary to expand the mode field such that the difference in the ultimate fundamental MFDs of the expander and the sapphire fiber rod are minimized. As well, the numerical aperture (NA) and the outer diameter of the expander should be closely matched to the rod waveguide in order to achieve efficient coupling of the LP01 mode of the expander to the LP01 mode supported by sapphire rod. The Gaussian shaped mode field launched by the expander that is used to excite the fundamental mode of the rod waveguide, must be large enough that the edges of the mode field extend to and are guided by the core-air interface of the sapphire rod. Launching with an expanded mode field with a MFD that is much smaller than the LP01 mode supported by the rod waveguide, will result in divergence of the mode field inside the fiber rod until it reaches the core-air interface of the rod resulting in coupling of the fundamental mode into higher order modes within the rod waveguide. As the narrowest standard sapphire optical fiber rods are typically 125-150 μm in diameter, down tapering alone cannot sufficiently expand the mode field from a single mode fiber.
High factors of mode field expansion are possible when a fiber is ‘up’ tapered as taught by Amitay et al in U.S. Pat. No. 4,737,004. Although this prior art single mode expansion techniques provide a useful function by facilitating expansion of the fundamental single mode of the fiber to arbitrarily large mode fields, it is not easily manufactured from commercially available fibers. These expanders are made by maintaining some of the remaining tapered section of single mode fibers that are drawn from fiber performs through preferential cleaving along the length of the fiber as it is being drawn. As such it is not ideally suited for mass production. As a cladding is present on the fiber ‘up’ taper, as taught by Amitay in U.S. Pat. No. ‘004’, the numerical aperature (NA) of the expander is similar to that of single mode fiber, which is typically ˜0.1.
For optimal bi-directional coupling, the numerical aperture (NA) of the LP01 mode of the expander and the NA of the LP01 mode of the target waveguide into which the fundamental mode is launched, need to be substantially matched in order to avoid excess coupling loss. For FBG sensors, the beam expansion technique needs to be bi-directional, meaning that as the fundamental mode exiting the SMF is subsequently expanded, similarly a large MFD fundamental mode received by the expander must be coupled into the single mode fiber of the expander when the mode field is propagating in the opposite direction. The NA of sapphire rod fibers is >1. It is likely then that a large MFD signal collected by an ‘up’ taper as taught by Amitay, would result in large amounts of excess loss due to NA mismatch.
It is an objective of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of mode field expanders that are compatible with high NA fiber rods absent a cladding.